1. Field of the Invention
The present invention relates to an improved working vessel such as a dredger or the like and more particularly to a working vessel of the type including a working apparatus at the one end part of the vessel body and a turning shaft member for the vessel body at the other end part of the body.
2. Description of the prior art
Hitherto known pump suction type dredgers are typically illustrated in FIGS. 1 to 4.
One of the conventional dredgers will be described first with reference to FIGS. 1 and 2. As will be apparent from the drawings, a main spud 1 and an auxiliary spud 2 are arranged at the one end part of the vessel body, whereas working means comprising a ladder 6 and swing wires 7 is arranged at the other end part of the vessel body so as to carry out a dredging operation. However, this spud type dredger has a drawback in that trouble often takes place in the form of breakage of the spuds 1 and/or 2 due to bending moment arising from surging of the vessel body under the influence of rushing waves. It is known from hitherto encountered experience that an operational limit is defined by a wave height in the range of 0.4 to 0.5 m in a sea area where a so-called swell having an average period longer than 7 seconds is developed. When a dredging operation is to be carried out under more critical climate condition, a Christmas tree type dredger as illustrated in FIGS. 3 and 4 is employed. In the Christmas tree type the dredger includes an anchoring mechanism 3 and wires 4 at the one end part of the vessel body so as to maintain the dredger at a predetermined position and the dredging operation is carried out while the vessel body is caused to turn to the left and right about the Christmas tree type anchoring mechanism 3 as a center of turning movement with the aid of swing wires 7 adapted to be reeled or unreeled, said swing wires 7 extending from the fore part of the ladder 6 in both the leftward and rightward directions. It is found that the Christmas tree type dredger can be operated more satisfactorily than the spud type one under any critical climate condition on the sea but the former has drawbacks that the vessel body is maintained with a predetermined position at less accuracy than that of the latter and therefore a precise dredging operation fails to be carried out.
One of factors causing reduced efficiency and an operational limit of the dredging operation with the conventional pump suction type dredgers under the influence of rushing waves is attributable to the fact that a cutter head at the foremost end of the ladder is largely displaced in both the vertical and longitudinal directions due to displacement of the vessel body under the influence of rushing or roaring waves, resulting in reduced capacity of suction and occurance of damage or injury on the dredging apparatus including cutter, cutter shaft and others.
It is believed that a cause for degraded properties of the conventional pump suction type dredger consists mainly in reduced strength of the anchoring mechanism, low resistibility against high waves and a large extent of displacement of the cutter head in both the vertical and longitudinal directions. To resolve the foregoing problems there was proposed an improved system in which the vessel body is held on the water surface with the aid of a jack-up apparatus. However, the proposed system has drawbacks that a dredger is manufactured at a very expensive cost and operation of the dredger is largely dependent on geometrical conditions at the bottom of the sea.